Abstract:

The abundance of birds recorded in the North American Breeding Bird Survey decreased by up to 18 percent between 1966 and 2005. The abundance of US and Canadian resident species decreased by 30 percent, and that of migrants within the United States and Canada decreased by 19 percent. By contrast, Neotropical migrants increased by up to 20 percent. Land-cover changes in northern latitudes therefore seem more consequential for bird populations than those occurring in Neotropical habitats. Lower abundances were most marked for resident breeding birds that used open, edge, and wetland habitats, the environments most affected by human disturbances—particularly urban sprawl—in northern latitudes. The abundance of resident and migrant forest-dwelling birds increased (although trends varied from species to species), with the increases seeming to follow the 20th-century expansion of forest area in northern latitudes, rather than the loss of Neotropical forests. The geographic footprint of changes in bird abundance linked to habitat changes in North America may thus be extending southward, with negative effects on birds that use open habitats and positive effects on forest birds.